ZIF-67, constructed by cobalt ions and MeIm, is of isostructural SOD zeolitic topology and same theoretical pore aperture to ZIF-8.17 Therefore, well-intergrown ZIF-67 polycrystalline membrane is also expected to exhibit effective separation for propylene/propane mixtures. However, to the best of our knowledge, pure ZIF-67 polycrystalline membrane with effective performance for propylene/propane separation is not reported. Very recently, a well-intergrown ZIF-67 polycrystalline membrane was fabricated by Jeong et al. through the hetero-epitaxial growth on ZIF-8 seeds rather than on ZIF-67 seeds.18 We believe that the zinc element in the deposited seeds plays an important role in controlling the grain boundary structure of polycrystalline ZIF-67 membranes. However, the exploration of the exact role of zinc ions in the ZIF-67 framework using this synthesis strategy is unreliable.
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